The invention described in detail below will be understood best in light of the following discussion of the state of the art and the problem that the invention solves.
Electronic databases provide a solution for long-term storage needs, as well as rapid search and retrieval of information. A database is merely a collection of data stored together and organized for rapid search and retrieval. In general, a database stores information internally as one or more “tables.” A table is a collection of “records” (also called “rows”), and a record is a collection of “fields” (also called “columns”). Every record in a given table must have the same number of fields. Fields contain discrete data values.
In general, most databases are either hierarchical or relational. A database that implements a hierarchical data model links data together by embedding pointers within the data. The links in a hierarchical database are static. Static links decrease the complexity of data access, but limit flexibility. The relational model evolved as a means to improve the flexibility of data access. Relational models do not rely on static links. Rather, relational models allow a user to link data together dynamically. Dynamic links, in turn, enable a user to tailor data access to specific needs.
Typically, a database management system (DBMS) provides a user interface (UI) through which a user may access the, information contained in a database. A DBMS commonly implements some form of command language that allows a user to construct a request for specific information. The Structured Query Language (SQL) is one example of a command language that has gained widespread acceptance in the art. A command that requests information from a database is referred to generally as a “query.” A DBMS responds to a user's query by returning the requested information in a “result set.” A result set may be displayed on the user's screen as a series of rows and columns, or may be saved to a file for future use or further processing.
Similarly, a DBMS usually provides an application program interface (API) through which other computer programs may access the information contained in a database. Although the implementations vary from one DBMS to another, many DBMS APIs make use of a command language similar to the command language that the DBMS implements in the UI. APIs commonly allow a programmer simply to embed the same command language within the program. And just as a DBMS returns a result set to a user in response to a user's query, a DBMS returns a result set to a program in response to the program's query.
A query generally consists of a statement that identifies what information should be retrieved and where that information can be found. Most DBMSs require a query to state (at a minimum) from which table or tables to retrieve the data (a “from-clause”), which fields to retrieve (a “select-clause”), and the selection criteria (a “where-clause”). If the query identifies two or more tables in a relational database, the query must also state the relation between the tables.
Regardless, though, of whether a DBMS is responding to a user's query or a program's query, the query statement determines the arrangement (or “shape”) of the result set returned by a DBMS. In particular, the result set presents the fields in the order that they were stated in the select-clause. A user, of course, usually knows the order in which the fields were stated in the select-clause and, thus, knows the shape of the result set in advance. Similarly, a programmer that embeds query statements in a program knows the shape of the result set in advance. Advance knowledge of the result set's shape allows a programmer to build a program that anticipates a particular shape and processes the result set accordingly.
There are many occasions, however, when it is desirable to build programs that can interact with a database without knowing the shape of a result set in advance. For example, many programs are built to provide a layer of abstraction between a database and an end-user program. Programs that provide this layer of abstraction are commonly referred to as “middleware.” In order to maximize usability and flexibility, a middleware program needs to be able to accommodate queries that come from an end-user program, or even end-users themselves. Thus, a middleware programmer usually will not know the result shape in advance, and must be able to discover dynamically the result shape as the program is executing. One possible method of dynamic shape discovery is to parse the query statement within the middleware program. Parsing a query statement, though, is a complicated process and requires substantial additional programming effort. Furthermore, the parsing method would result in slower execution and redundant parsing, since a DBMS must also parse the query statement. Therefore, a need exists for a method of dynamically discovering the shape of a result set, without the development and use of a complicated and time consuming language parser.